What is bipedalism?

Hominids include the great apes (orangutans, gorillas and chimpanzees) and humans, while Hominins include Humans and their fossil ancestors. Two main features that separate Hominins from the apes is that they are relatively hairless, and they are full bipedal (walk on two legs with their forelimbs free).

Bipedalism is the method of locomotion that sets Hominins apart from the other Hominids; humans are fully bipedal whereas apes are quadrupedal knuckle walkers. Hominins have many adaptations compared to apes that enable them to have bipedal locomotion, and these include adaptations to;

• Skull

• Vertebral column

• Pelvis

• Femure

• Knee

• Foot

• Arm-to-leg ratio

• Centre of gravity

Adaptations for Bipedalism: Skull

Hominid

• Foramen magnum is found at the back of the base of the skull

• More prognathic (jaw protruding more)

• Heavy U-shaped jaw with large canines with diastema

Hominin

• Foramen magnum at the centre of the base of the skull

• Flatter face (less prognathic)

• Lighter horeshoe shaped jaw with smaller canines with no diastema

Advantage to the Hominin

• Allows the skull to balance on the vertebral column with little effort

• This reduces the requirement for large neck muscles

Adaptions for Bipedalism: Vertebral Column

Hominid

• Single curve (C shape)

• Non-wedge shaped lumbar vertebrae

• Large processes on neck vertebrae

Hominin

• S-shaped curve

• Wedge shaped lumbar vertebrae

• Small processes on neck vertebrae

Advantage to the Hominin

• Allows for centre of gravity to pass through the knee and foot; skull can balance on the column with little muscle support

• Forms lumbar curve to help balance the body

• Allows for increased neck movement

Adaptations for Bipedalism: Pelvis

Hominid

• Long and narrow and titled forward

Hominin

• Short, broad, bowl shaped and vertical

Advantage to the Hominin

• Short and broad pelvis provides surface for attachment of leg muscles; this allows for the movement of the legs while keeping the upper body erect

• Bowl shaped to support abdominal organs (support developing foetus during pregnancy

• Vertical angle allows alignment with upright vertebral column

Adaptations for Bipedalism: Femur

Hominid

• Femurs come straight down from pelvis

• Shorter femur

Hominin

• Modified acetabulum and longer ball shaft on femur

• Femurs angle in from pelvis

• Longer femur

• Femur is more robust

Advantage to the Hominin

• Creates a carrying angle and brings the knees closer together, thus making it easier to transfer weight from one foot to another when walking; this allows humans to have a striding gait instead of swaying from side to side

• Allows for a longer stride

Adaptations for Bipedalism: Knee

Hominid

• Single hinge

Hominin

• Two-part robust hinge joint with ligaments

Advantage to the Hominin

• Robust (load bearing)

• Centre of gravity falls in front of the knee; this forces the knee backwards but is stopped by ligaments; little energy is required for the standing position

Adaptations for Bipedalism: Foot

Hominid

• Longitudinal arch

• Opposable big toe

• Small calcaneus

Hominin

• Longitudinal and transverse arches

• Large big toe with it alongside others

• Large robust calcaneus

Advantage to the Hominin

• Produces a spring action and acts as a shock absorber when walking

• Acts to give a push off (lever) when walking as energy is transferred through it

• Bears impact of bodies weight (load bearing)

Adaptations for Bipedalism: Arm-to-leg ratio, Centre of gravity

Arm-to-leg ratio

Hominid

• Arms longer than legs

• Centre of gravity is higher within the body (chest level)

Hominin

• Legs longer than arms

• Centre of gravity is lower within the body (located in the pelvis)

Advantage to the Hominin

• Allows long striding gait

• Increases stability while bipedal and ability to stand upright

Centre of gravity

Hominid

• Centre of gravity falls in front of the knee and feet

• Centre of gravity is higher within the body (located above the pelvis)

Hominin

• Centre of gravity falls through the front of the knee and centre of the feet

• Centre of gravity is lower within the body (located in the pelvis)

Advantage to the Hominin

• Increases stability while bipedal and ability to stand upright

• Upright stance is achieved with minimal effort